On Tuesday 7 January, Coptic Orthodox Christians – who comprise 90 percent of Egypt's Christians – will break their 43-day fast and celebrate Christmas. The festival comes almost two weeks after most Western denominations, including Catholics and Protestants, held their celebrations on 25 December.

Ahram Online asked Bishop Abram of the Fayoum Diocese to explain why there was a difference in the dates; he stressed that the difference in fact results from the use of different calendars, not from any underlying theological dispute.

Although the exact date of Jesus' birth was -- and remains -- unknown, within the first few centuries after his death, churches around the world agreed to celebrate the nativity of Jesus Christ on 25 December (29 Kiahk in the Coptic calendar), most probably to replace the pagan feast celebrating the Roman winter solstice which continued to be observed until then.

Bishop Abram argues that celebrating the birth of Christ, considered by Christians to be "the light of the world" is also astronomically apt, since night-time begins to shorten and daylight to lengthen in the middle of December.

The different dates of the celebration in the modern period are a result of a change in calendar; while Western churches follow the Gregorian calendar, Orthodox churches continue to follow the older Julian calendar (named after Julius Caesar who introduced it in 46 BC), which in turn is in line with the ancient Coptic calendar.

Until the Julian calendar, the date of 25 December and 29 Kiahk in the Coptic calendar happened on the same day each year. But the introduction of the Gregorian calendar changed this alignment.

In the 16th century, Pope Gregory XIII of the Roman Catholic Church had his astronomers study the calendar, and they realised that the Julian year was 11 minutes shorter than the actual solar year. Over time, the 11 minutes had added up – the equivalent of three extra days being added to the calendar every 400 years. The calendar date was becoming out of sync with the solar year.

To correct this, Pope Gregory's recalculated the whole system and cut out the extra days for his new Gregorian calendar. As a result of the difference, the Julian calendar is now 13 days behind the Gregorian calendar, and 25 December in the Julian system falls on 7 January in the Gregorian system.

Although the Gregorian calendar was adopted by Khedive Ismail in 1875, the Coptic Orthodox Church has continued to use the older calendar, and 29 Kiahk (or 25 December in the Julian calendar) falls, under the Gregorian system, on 7 January.